As a conventional solid polymer electrolyte fuel cell (hereinafter, it may be simply referred to as a fuel cell), a plane-shaped microcell has mainly been developed, wherein the plane-shaped microcell is produced by disposing catalyst layers to be an anode and a cathode on one surface and the other surface of a plane-shaped solid polymer electrolyte membrane respectively and gas diffusion layers on both sides of the obtained plane-shaped membrane electrode assembly (an assembly comprising the electrolyte membrane and the electrode layers) respectively, further interposing the assembly between plane-shaped separators. A microcell is a minimum power-generation unit of a fuel cell, and a fuel cell stack is obtained by stacking plurality of such plane-shaped microcells.
In order to improve power density of the solid polymer electrolyte fuel cell, a proton conductive polymer membrane with a very thin membrane thickness is used as the solid polymer electrolyte membrane. The membrane thickness is often 10 μm or less, and though a further thin electrolyte membrane is used for improvement of power density, a thickness of the microcell cannot extremely be reduced beyond conventional ones. Similarly, a catalyst layer, a gas diffusion layer, a separator or the like are also undergoing their thickness reduction. However, improvement of power density per unit volume is limited even by the thickness reduction of all members. Hence, demands for compact size may not be fully satisfied in future.
As the separator mentioned above, a sheet-like carbon material which is excellent in corrosion resistance is generally used. The carbon material is expensive itself. In addition, a surface of the separator is often subject to a fine work for forming grooves to be a gas channel in order to evenly supply the fuel gas and the oxidant gas over the entire face of the plane-shaped membrane electrode assembly (an assembly of the electrolyte membrane and the electrode layers). Hence, the separator becomes too expensive due to such fine work and raises a manufacturing cost of the fuel cell.
In addition to the above described problems, the plane-shaped microcell has many problems such that a safe sealing of a periphery of plural microcells which are stacked in order to prevent leakage of the fuel gas and the oxidant gas from the above mentioned gas channel is technically difficult, and such that the power generation efficiency is lowered due to distortion or deformation of the plane-shaped membrane electrode assembly (an assembly of the electrolyte membrane and the electrode layers).
For instance, in order to downsizing the fuel cell described above and to increase a reaction area for generation per unit volume regarding power density, all constituent members described above of the fuel cell are necessary to undergo their thickness reduction. However, in a conventional fuel cell having a plate-like structure, reducing the thickness of each constituent member below a certain value is not preferable from aspects of function and strength and is approaching its limit of design. For example, there are problems such that a commonly used Nafion (product name; manufactured by: DuPont) with the thickness below a certain value has too high gas permeability and produces gas cross leak so as to cause reduction in generated voltage. Hence, improving power density per unit volume above a certain level is structurally difficult in a conventional fuel cell having a plate-like structure.
Accordingly, there are studies to increase power density by composing a fuel cell using a hollow-shaped (for instance, a tube-shaped) membrane electrode assembly, wherein an electrolyte membrane, electrode layers and so on are layered on inner and outer surfaces of a hollow fiber for a hollow-shaped microcell (hereinafter, it may be simply referred to as a hollow-shaped cell). Such a hollow-shaped membrane electrode assembly and a hollow-shaped cell using thereof can significantly improve power density per unit volume in comparison to a conventional fuel cell having a plate-like structure by densely disposing many tubes having small diameters (see Japanese Patent Application Laid-open (JP-A) No. 2002-124273 and JP-A No. Hei. 7(1995)-296840).
Such a fuel cell comprising a hollow-shaped cell does not require a member, which is equivalent to a separator used for a plane-shaped microcell since a hollow of the cell functions as a gas channel. Also, forming an extra gas channel is not necessary since different types of gasses are respectively supplied over the inner and outer surfaces of the hollow-shaped cell. Hence, reduction in production costs is possible. In addition, since the microcell has a three-dimensional form, a specific surface area with respect to volume can be enlarged in comparison to that of a plane-shaped microcell, and improvement in generation power density per volume is expected.
Currently, there are various attempts to improve power density per unit volume of such a tube-shaped membrane electrode assembly and a hollow-shaped cell using thereof.
To obtain desirable output voltage and current, a fuel cell using a hollow-shaped cell has a structure that plurality of hollow-shaped cells are electrically connected and made into a module (a group of hollow-shaped cells) with collector materials, and two or more modules are connected in series and/or in parallel.
In such a module, an adequate number of heat exchanging members (hereinafter, it may be referred to as “cooling pipe”) to cool/heat hollow-shaped cells are disposed in parallel with hollow-shaped cells.
This is because types of electrolytes allow the hollow-shaped cell to determine the most suitable temperature range for electrochemical reaction similarly as the microcell comprising a membrane electrode assembly having a plane-like shape (for instance, the temperature is about 100° C. in the case of perfluorocarbon sulfonate membrane). In order to improve power generating performance, the hollow-shaped cell is subject to cooling so as to fix the temperature of the cell in a predetermined range of temperature. On the other hand, from the viewpoint of improving the start-up performance at low temperature of a fuel cell, the hollow-shaped cell requires to be heated when the fuel cell starts. For example, Japanese translation of PCT international application No. 2004-505417 discloses a technique of bundling a plurality of hollow-shaped cells (microcells) to form a modular electrochemical cell assembly and disposing tube-shaped conductive pipes in parallel with and between the microcell bundles. According to the document, the technique enables removal of a large quantity of heat generated by the microcell bundles.
At both ends of the module, a gas manifold to supply hydrogen gas into a hollow of the hollow-shaped cell and a cold water manifold to supply heating medium into a heat exchanging member are provided. Further, a current collector member to collect electric charge generated at each hollow-shaped cell is provided. Hydrogen supplied to the module through the gas manifold on the inlet side is used for an electrochemical reaction while passing through the channel in the hollow of each hollow-shaped cell. Hydrogen or the like which are not used for the electrochemical reaction are collected through the gas manifold on the outlet side. Sealing is applied to the part where a hollow-shaped cell contacts each manifold, which is referred to as a sealing portion. A fuel cell has a structure that only the sealing portions support hollow-shaped cells so that the sealing portions hold the whole weight of hollow-shaped cells. Also, the sealing portions are mostly affected by distortion due to the difference in thermal expansion between the hollow-shaped cell and the manifold. For these reasons, there is a problem that the sealing portions are particularly breakable.
In a conventional fuel cell, as exemplified above, a plurality of linear hollow-shaped cells having the same length as that of a heat exchanging member are disposed in parallel. When one linear hollow-shaped cell is broken due to damage to sealing portions or any other events causing gas leakage, a module containing the damaged hollow-shaped cell becomes unusable. To avoid such a problem, decreasing the number of sealing portions per cell volume is effective.
The number of sealing portions per cell volume can be reduced by making the length of each hollow-shaped cell longer so as to decrease the number of the hollow-shaped cells. However, there are problems that it is difficult to handle a hollow-shaped cell when the hollow-shaped cell is made longer keeping a linear form and strength in the middle portion of the hollow-shaped cell in the axial direction becomes unstable.
The present invention has been achieved in light of the above-mentioned circumstances, and a main object of the present invention is to provide a hollow-shaped membrane electrode assembly for a fuel cell which is capable of improving power density per unit volume and easy to handle.
Another object of the present invention is to provide a fuel cell using a hollow-shaped membrane electrode assembly for a fuel cell which is capable of improving power density per unit volume, wherein the number of sealing portions per cell volume can be reduced without sacrificing easiness of handling of the hollow-shaped cell using the hollow-shaped membrane electrode assembly and strength in the middle portion of the hollow-shaped cell in the axial direction.